Natural gas is a serious fuel candidate for mobile internal combustion engines because of its clean burning characteristics, its geographically wide availability, and its cost. Because natural gas, however, has a very low volumetric energy content at atmospheric pressure, special provisions must be employed to store a practical quantity of fuel energy on board mobile engine platforms. The two primary storage provisions are cryogenic liquefaction in thermally insulated tanks (known as liquid natural gas or LNG) or pressurization to about 3,000 psi in high pressure tanks (known as compressed natural gas). In addition, most mobile engine platforms have, heretofore, used conventional spark ignited otto cycle engines which are, as compared with diesel cycle engines, thermodynamically less efficient.
Mobile engine platforms require readily available sources of fuel to enable the user unfettered geographic movement. Liquid natural gas systems, in addition to adding substantial complexity to the mobile platform, lack a geographically widely-dispersed distribution system for refueling such mobile platforms. Accordingly, compressed natural gas systems for mobile platforms has found wide spread use because of a well developed distribution system for refueling and a relatively simple storage means for attachment to the mobile platforms.
Most compressed natural gas systems for mobile platforms have been employed in combination with spark ignited Otto cycle engines. Primary reasons for such use with Otto cycle engines rather than diesel cycle engines is that diesel cycle engines require injection into the utilizing engine combustion chamber at pressures in the 3,000 psi range due to the relatively high pressures realized within diesel cycle engines' combustion chambers as compared with Otto cycle engines' combustion chambers. Such higher pressure may, initially, be provided by the natural gas under pressure in its fuel storage reservoir as supplied by the refueling facilities. However, as the natural gas in the reservoir is consumed, the pressure of the natural gas remaining in such reservoir drops. Incorporating a high pressure compressor on a mobile platform suitable to pump the remaining natural gas to a suitably high pressure as its pressure (in the reservoir) drops to an unacceptably low level becomes a major consideration in terms of size and cost, especially in the case of a small mobile platform.
One attempt to provide the requisite injection pressure for natural gas in an internal combustion engine which is supplied with a variable pressure gaseous fuel supply is shown in U.S. Pat. No. 5,067,467 which issued Nov. 26, 1991. That invention used unit injectors having pressure intensifier apparatus driven by high pressure air supplied by the engine during its compression stroke. Such fuel system enables high pressure utilization of compressed natural gas stored in a high pressure reservoir but employs very complex structure and control apparatus. U.S. Pat. No. 4,765,304 issued Aug. 23, 1988 also employs fuel injectors driven by compressed air obtained from the combustion chambers of an associated internal combustion engine. U.S. Pat. No. 4,205,638 issued Jun. 3, 1980, illustrates the use of a free piston unit to use energy stored in a working medium to compress a fuel/air mixture injected into the combustion chamber of an engine. U.S. Pat. No. 3,682,146 issued Aug. 8, 1972, illustrates a system of fuel injection in a diesel cycle precombustion chamber in which a tank and jet pump are connected to the combustion chamber of an engine associated with the fuel system.
None of the aforementioned inventions have provided an altogether satisfactory solution to maintaining the pressure of natural gas or other compressible fuel housed within a reservoir at a pressure permitting its use in a diesel cycle engine when the natural gas is being withdrawn from the reservoir. The present invention is directed to overcoming one or more of the problems set forth above.